1. Field
The following description generally relates to a technology for Radio Frequency IDentification (RFID), and more particularly to a technology for RFID tags which has multiple antennas to provide uplink communications through wireless communications of a backscatter modulation scheme.
2. Description of the Related Art
Radio Frequency IDentification (RFID) is a wireless technology to read or record information on tags by using radio frequencies, and is used to identify, track, or manage tagged items, animals, or people. Such RFID system has unique identification information, and is composed of a transponder or an RFID tag attached to objects or people, an interrogator or an RFID reader to read information on an RFID tag or write information thereon, a database, a network, and the like.
In the RFID system, the RFID reader transmits an RF signal with a carrier signal to the RFID tag, and the RFID tag generates its own power by using the carrier signal received from the RFID reader, uses the power to transmit information, generated by encoding and modulating stored data, as an uplink signal to the RFID reader. Such wireless communication in the RFID system is called a backscatter modulation, which is mainly used in a passive or semi-passive RFID system in an Ultra High Frequency (UHF) range of 860 MHz to 960 MHz.
The RFID tag is generally provided with a small flexible antenna to be easily attached to products. However, it is known that when facing a channel environment such as multipath fading, the UHF-band RFID system may increase recognition errors, that is, may reduce a recognition rate. In order to overcome the drawbacks, a method of using two or more antennas with an orthogonal polarization has been proposed. For example, an RFID tag with two antennas may doubly harvest DC power from electromagnetic waves of an RFID reader in downlink, and may doubly backscatter information in uplink. As a result, better performance in terms of a recognition rate, orientation sensitivity, and the like, may be achieved when compared with a single antenna RFID tag.
Further, in the case of using a multiple antenna RFID tag with two or more antennas, Multiple Input and Multiple Output (MIMO), space-time block codes (STBC), dual-speed Amplitude Shift Keying (ASK), and the like may be applied. In these schemes, information is transmitted by changing either or both of amplitudes and phases of an uplink signal transmitted by backscattering, such that the schemes may improve the speed of uplink communications.
However, in order to improve the speed in uplink communications in the RFID system while preventing reduction in a recognition rate, multiple antennas should be in an almost similar environment of electromagnetic waves, but in practice it is impossible that antennas are in the same or similar environment of electromagnetic waves all the time. For this reason, some data may be lost during transmission, resulting in recognition errors of an uplink signal. For example, in the case where a specific antenna, among a plurality of antennas, is in a bad environment of electromagnetic waves, an RFID reader may not accurately recognize an uplink signal transmitted by an RFID tag, or may mistakenly recognize an uplink signal as some other signal. Even when the speed of uplink communications is increased by using a new technology, it is undesirable that a recognition rate of an uplink signal is reduced.